The present invention relates to cellular telecommunications, more particularly to cellular telecommunications employing both full frequency duplex (FDD) transmissions and time division duplex (TDD) transmissions, and even more particularly to methods and apparatuses that, inter alia, enable a user equipment (UE) to determine whether the transmissions of a neighboring cell are uplink or downlink transmissions.
In the forthcoming evolution of the mobile cellular standards like the Global System for Mobile Communication (GSM) and Wideband Code Division Multiple Access (WCDMA), new transmission techniques like Orthogonal Frequency Division Multiplexing (OFDM) are likely to occur. Furthermore, in order to have a smooth migration from the existing cellular systems to the new high capacity high data rate system in existing radio spectrum, a new system has to be able to utilize a bandwidth of varying size. A proposal for such a new flexible cellular system, called Third Generation Long Term Evolution (3G LTE), can be seen as an evolution of the 3G WCDMA standard. This system will use OFDM as the multiple access technique (called OFDMA) in the downlink and will be able to operate on bandwidths ranging from 1.4 MHz to 20 MHz. Furthermore, data rates up to and exceeding 100 Mb/s will be supported for the largest bandwidth. However, it is expected that 3G LTE will be used not only for high rate services, but also for low rate services like voice. Since 3G LTE is designed for Transmission Control Protocol/Internet Protocol (TCP/IP), Voice over IP (VoIP) will be the service that carries speech.
Transmissions from the system that are targeted to be received by a single user take place in what is termed a “unicast” mode of operation. Here, there is a single transmitter that communicates information to a single intended receiver. The LTE system is, however, additionally designed to support broadcast/multicast services, called Multimedia Broadcast/Multicast Service (MBMS).
The provisioning of broadcast/multicast services in a mobile communication system allows the same information to be simultaneously provided to multiple, often a large number of, mobile terminals, often dispersed over a large area corresponding to a large number of cells. FIG. 1 illustrates this point by showing a broadcast area 101 that comprises a number of cells 103. The broadcast/multicast information may be a TV news clip, information about the local weather conditions, stock-market information, or any other kind of information that, at a given time instant, may be of interest to a large number of users.
When the same information is to be provided to multiple mobile terminals within a cell it is often beneficial to provide this information as a single “broadcast” radio transmission covering the entire cell and simultaneously being received by all relevant mobile terminals rather than providing the information by means of individual transmissions to each mobile terminal (i.e., plural unicast transmissions).
As a broadcast transmission within a cell has to be dimensioned to operate under worst-case conditions (e.g., it needs to be able to reach mobile terminals at the cell border even though other mobile terminals may be quite close to the transmitter antenna), it can be relatively costly in terms of the resources (base station transmit power) needed to provide a given broadcast-service data rate. Alternatively, taking into account the limited signal-to-noise ratio that can be achieved at poor areas of reception within the cell (e.g. the cell edge), the achievable broadcast data rates may be relatively limited, especially when large cells are involved. One way to increase the broadcast data rates would then be to reduce the cell size, thereby increasing the power of the received signal at the cell's edge. However, such an approach would increase the number of cells needed to cover a certain area and would thus obviously be undesirable from a cost-of-deployment point-of-view.
However, as discussed above, the provisioning of broadcast/multicast services in a mobile communication network typically occurs when identical information is to be provided over a large number of cells. In such cases, the resources (e.g., base-station transmit power) needed to provide a desired broadcast data rate can be considerably reduced if, when detecting/decoding the broadcast data, mobile terminals at the cell edge can utilize the received power from multiple broadcast transmissions emanating from multiple cells.
One way to achieve this is to ensure that the broadcast transmissions from different cells are truly identical and transmitted mutually time-aligned. Under these conditions, the transmissions received by user equipment (UE) (e.g., a mobile terminal) from multiple cells will appear as a single transmission subject to severe multi-path propagation. The transmission of identical time-aligned signals from multiple cells, especially when utilized to provide broadcast/multicast services, is sometimes referred to as Single-Frequency-Network (SFN) operation or Multicast-Broadcast Single Frequency Network (MBSFN) operation.
When multiple cells transmit such identical time-aligned signals, the UE no longer experiences “inter-cell interference” from its neighbor cells, but instead experiences signal corruption due to time dispersion. If the broadcast transmission is based on OFDM with a cyclic prefix that covers the main part of this “time dispersion”, the achievable broadcast data rates are thus only limited by noise, implying that, especially in smaller cells, very high broadcast data rates can be achieved. Furthermore, the OFDM receiver does not need to explicitly identify the cells to be soft combined. Rather, all cells whose transmissions fall within the cyclic prefix will “automatically” contribute to the power of the UE's received signal.
In each of the unicast and multicast modes, the LTE physical layer downlink transmission is based on OFDM. The basic LTE downlink physical resource can thus be seen as a time-frequency grid as illustrated in FIG. 2, in which each so-called “resource element” corresponds to one OFDM subcarrier during one OFDM symbol interval.
As illustrated in FIG. 3, the downlink subcarriers in the frequency domain are grouped into resource blocks, where each resource block consists of twelve consecutive subcarriers for a duration of one 0.5 ms slot (7 OFDM symbols when normal cyclic prefixes are used (as illustrated) or 6 OFDM symbols when extended cyclic prefixes are used), corresponding to a nominal resource-block bandwidth of 180 kHz.
The total number of downlink subcarriers, including a DC-subcarrier, thus equals NC=12·NRB+1 where NRB is the maximum number of resource blocks that can be formed from the 12·NRB usable subcarriers. The LTE physical-layer specification actually allows for a downlink carrier to consist of any number of resource blocks, ranging from NRB−min=6 and upwards, corresponding to a nominal transmission bandwidth ranging from around 1.25 MHz up to 20 MHz. This allows for a very high degree of LTE bandwidth/spectrum flexibility, at least from a physical-layer-specification point-of-view.
FIGS. 4a and 4b illustrate the time-domain structure for LTE downlink transmission. Each 1 ms subframe 400 consists of two slots of length Tslot=0.5 ms (=15360·TS, wherein each slot comprises 15,360 time units, TS). Each slot then consists of a number of OFDM symbols.
A subcarrier spacing Δf=15 kHz corresponds to a useful symbol time Tu=1/Δf≈66.7 μs (2048·TS). The overall OFDM symbol time is then the sum of the useful symbol time and the cyclic prefix length TCP. Two cyclic prefix lengths are defined. FIG. 4a illustrates a normal cyclic prefix length, which allows seven OFDM symbols per slot to be communicated. The length of a normal cyclic prefix, TCP, is 160·TS≈5.1 μs for the first OFDM symbol of the slot, and 144·TS≈4.7 μs for the remaining OFDM symbols.
FIG. 4b illustrates an extended cyclic prefix, which because of its longer size, allows only six OFDM symbols per slot to be communicated. The length of an extended cyclic prefix, TCP−e, is 512·TS≈16.7 μs.
It will be observed that, in the case of the normal cyclic prefix, the cyclic prefix length for the first OFDM symbol of a slot is somewhat larger than those for the remaining OFDM symbols. The reason for this is simply to fill out the entire 0.5 ms slot, as the number of time units per slot, TS, (15360) is not evenly divisible by seven.
When the downlink time-domain structure of a resource block is taken into account (i.e., the use of 12 subcarriers during a 0.5 ms slot), it will be seen that each resource block consists of 12·7=84 resource elements for the case of normal cyclic prefix (illustrated in FIG. 3), and 12·6=72 resource elements for the case of the extended cyclic prefix (not shown).
Another important aspect of a terminal's operation is mobility, which includes cell search, synchronization, and signal power measurement procedures. Cell search is the procedure by which the terminal finds a cell to which it can potentially connect. As part of the cell search procedure, the terminal obtains the identity of the cell and estimates the frame timing of the identified cell. The cell search procedure also provides estimates of parameters essential for reception of system information on the broadcast channel, containing the remaining parameters required for accessing the system.
To avoid complicated cell planning, the number of physical layer cell identities should be sufficiently large. For example, systems in accordance with the LTE standards support 504 different cell identities. These 504 different cell identities are divided into 168 groups of three identities each.
In order to reduce the cell-search complexity, cell search for LTE is typically done in several steps that make up a process that is similar to the three-step cell-search procedure of WCDMA. To assist the terminal in this procedure, LTE provides a primary synchronization signal and a secondary synchronization signal on the downlink. This is illustrated in FIG. 5, which illustrates the structure of the radio interface of an LTE system. The physical layer of an LTE system includes a generic radio frame 500 having a duration of 10 ms. FIG. 5 illustrates one such frame 500 for an LTE Frequency Division Duplex (FDD) system. Each frame has 20 slots (numbered 0 through 19), each slot having a duration of 0.5 ms which normally consists of seven OFDM symbols. A subframe is made up of two adjacent slots, and therefore has a duration of 1 ms, normally consisting of 14 OFDM symbols. The primary and secondary synchronization signals are specific sequences, inserted into the last two OFDM symbols in the first slot of each of subframes 0 and 5. In addition to the synchronization signals, part of the operation of the cell search procedure also exploits reference signals that are transmitted at known locations in the transmitted signal.
Furthermore, LTE is defined to be able to operate in both FDD mode as well as in Time Division Duplex (TDD) mode. Within one carrier, the different subframes of a frame can either be used for downlink transmission of for uplink transmission. FIG. 6a illustrates the case for FDD operation, wherein pairs of the radiofrequency spectrum are allocated to users, one part for uplink transmissions, and the other part for downlink transmissions. In this operation, all subframes of a carrier are used for either downlink transmission (a downlink carrier) or for uplink transmission (an uplink carrier).
By comparison, FIG. 6b illustrates the case for TDD operation. It will be observed that in this operation, the first and sixth subframe of each frame (i.e., subframes 0 and 5) are always assigned for downlink transmission, while the remaining subframes can be flexibly assigned to be used for either downlink or uplink transmission. The reason for the predefined assignment of the first and sixth subframe for downlink transmission is that these subframes include the LTE synchronization signals. The synchronization signals are transmitted on the downlink of each cell and, as explained earlier, are intended to be used for initial cell search as well as for neighbor-cell search.
FIG. 6b also illustrates the flexibility that LTE provides in assigning uplink and downlink subframes during TDD operation. This flexibility allows for different asymmetries in terms of the amount of radio resources (subframes) assigned for downlink and uplink transmission, respectively. For example, an approximately symmetric carrier 601 can be created, as can an asymmetric carrier with a downlink focus 603 (i.e., more downlink subframes than uplink subframes), and an asymmetric carrier with an uplink focus 605 (i.e., more uplink subframes than downlink subframes).
As the subframe assignment needs to be the same for neighbor cells in order to avoid severe interference between downlink and uplink transmissions between the cells, the downlink/uplink asymmetry cannot vary dynamically on, for example, a frame-by-frame basis. However, it can be changed on a slower basis to, for example, match different traffic characteristics such as differences and variations in the downlink/uplink traffic asymmetry.
In LTE, a measure of the Reference Signal Received Power RSRP is used for handover measurements. This means that the mobile terminal needs to measure RSRP on the serving cell as well as on those neighboring cells that have been detected by the cell search. RSRP is defined as the average signal power of the Node B's transmitted (i.e., downlink) Reference Symbols or Signals (RS). The RSs are transmitted from the Node B from each of possibly 1, 2 or 4 transmit antennas, on certain resource elements (RE) in the time-frequency grid. For example, in LTE the resource elements are transmitted on every sixth subcarrier in OFDM symbol number 0 and in either symbol number 3 (when long CPs are used) or symbol number 4 (when short CPs are used) in every slot (consisting of either 6 or 7 OFDM symbols, depending on whether long or short CPs are being used). Furthermore, the RS in symbol number 3/4 is offset by three subcarriers relative to the RS in the first OFDM symbol.
In order to arrive at an RSRP measurement that is truly representative of the signal conditions, the UE needs to average a number of measurements obtained over a number of slots (and subframes). For FDD operation, this can easily be done because the downlink and uplink transmissions occur on separate carriers, and hence all subframes of the downlink carriers can be used for generating an RSRP estimate.
However, for TDD operation, the uplink and downlink transmissions share the same carrier frequency, so not all of the subframes can be used. To complicate matters, the uplink/downlink configuration for different neighboring cells could—in the general case—be different. The uplink/downlink configuration of a newly detected cell (i.e., a cell that has just been detected as a potential handover candidate by the cell search procedure) is, at the time of detection, unknown to the UE. This information is conventionally first made known to the UE at the time of handover to that cell.
Accordingly, the UE is conventionally required to rely on RSs transmitted in only those subframes that are guaranteed to be associated with downlink transmissions (e.g., synchronization subframes 0 and 5 in LTE, as illustrated in FIG. 6b). Being limited to the RSs from only these slots results in a noisy RSRP (or similar) measurement, so longer averaging time is needed to generate a useful value, thereby delaying the handover procedure.
However, in a typical TDD uplink/downlink configuration, there are more downlink subframes than just the synchronization subframes. A super-frame in LTE is 10 ms divided into ten 1 ms subframes, two of which are sync subframes (see, e.g., FIG. 5). Typically, the uplink/downlink configuration is 40/60 or even 30/70, so there are actually more downlink subframes (and hence more RSs) available than just the RSs included in the synchronization subframes (corresponding to a downlink/uplink allocation of 20/80).
Therefore there is a need for methods and apparatuses that are able to detect the uplink/downlink configuration in TDD operation for neighboring cells at the time those cells are first detected, in order to improve the RSRP (or similar) measurement performance.